Light source

ABSTRACT

A light source, in particular incandescent lamp, with a bulb ( 1 ), and filament ( 2 ) arranged in the bulb ( 1 ), and a heating device ( 3 ) for the filament ( 2 ), the filament ( 2 ) emitting both visible light and heat radiation, is designed and constructed with respect to a high conversion efficiency between electric power and visible light output such that the heating device ( 3 ) includes a heating element ( 4 ) for the indirect heating of the filament ( 2 ).

BACKGROUND OF THE INVENTION

The invention relates to a light source, in particular incandescentlamp, with a bulb, a filament arranged in the bulb, and a heating devicefor the filament, the filament emitting both visible light and heatradiation.

Light sources of the described type have been known from practice for along time, and they exist in a large variety of designs and sizes. Inthis connection, for example, incandescent lamps are known as electricallight sources, in which it is common to bring a tungsten filament byelectrical Joule heat to a highest possible temperature. In thisprocess, a temperature radiation is generated. The light yield ofincandescent filaments considerably increases as the temperature rises.Besides that, also so-called nonthermal sources of radiation are known,for example, discharge lamps, such as inert gas-, mercury-, sodium-, andmetal halide discharge lamps in high-pressure and low-pressure designs.

All so far known, electrically operated types of light sources have thedisadvantage that they are very inefficient with respect to convertingelectric power into visible light output. The conversion barely exceeds30%. The largest portion of the consumed electric power is anuneconomical dissipation primarily in the form of heat.

A possibility of increasing the efficiency of known light sourcesconsists in that the heat radiated from the filament or glow wire, isreflected from the inner side of the bulb back to the filament or glowwire. As a result, the filament or glow wire undergoes a kind ofbackheating. This results in that after reaching the same filamenttemperature, less electric power will be needed than during a heatingwithout reflection. The visible light output, which is transmittedthrough the bulb, remains in this instance the same. In the ideal case,only that electric power will be needed, which corresponds to thevisible, emitted light output and to the thermal dissipation, which isabsorbed by the bulb. Thus, the conversion efficiency is improved by theportion of the reflected heat radiation. Theoretically, it would bepossible to increase with that the conversion efficiency to as much as75% or 140 lumens/watt, if one took as a basis the standard thermaldissipation of tungsten lamps of about 25%, and if one neglected theradiation absorption of a mirror coating on the inner side of the bulb.In this connection, for example, dielectric mirror coatings have anabsorption of typically 0.1%.

In the case of a mirror coating on the inner side of the bulb with areflecting power of, for example, 99.9%, statistically, every onethousandth photon in the material of the mirror coating will beabsorbed. In the case of a reflection of the radiation into the bulb,the photon flux may therefore undergo only 1000 reflections on the innerside of the bulb, until it is totally absorbed in the bulb. Theprobability that on its path of reflection, the photon flux strikes thefilament or glow wire and is there absorbed, is proportionate to theratio of the filament volume or the filament surface to the reflectingbulb volume or the reflecting bulb surface.

To achieve a highest possible backheating of the filament, it willtherefore be advantageous, when a large filament surface is present, sothat the photon flux strikes the filament and is there absorbed afterthe fewest possible reflections on the inner side of the bulb.

However, in this instance, it is disadvantageous that in the case of anenlarged filament surface, the electrical resistance of the filamentbecomes smaller, so that for reaching the filament temperature necessaryfor the light emission, a substantially greater current is needed in thefilament than in the case of a normal filament surface or normalfilament cross section. This may lead to safety problems for the user ofthe light source. In summary, there is a dilemma as to a largestpossible filament surface and the therefor required and disadvantageoushigh currents.

It is therefore an object of the present invention to describe a lightsource of the initially described type, which allows to achieve a highconversion efficiency with simple means and in a reliable manner.

SUMMARY OF THE INVENTION

The foregoing object is achieved by a light source which is designed andconstructed such that the heating device includes a heating element foran indirect heating of the filament.

In accordance with the invention, it has been recognized that thedevelopment of a separate heating element for the filament accomplishesthe foregoing object in a surprisingly simple manner. In this instance,the filament is indirectly heated by the heating element, which offersthe great advantage that the filament may be configured irrespective ofits electrical resistance behavior. As a result, it is possible torealize a large-surface filament, which exhibits a high absorptive powerfor heat radiation, which is reflected from the inner side of the bulb.The device, which is needed for heating the filament may be realizedindependently of the configuration of the filament. Consequently, it isalso possible to realize a heating device, which operates with electriccurrents, which can be safely managed. An electrical contact between theheating device and the filament is no longer needed.

Thus, the light source of the present invention denotes a light source,which allows to achieve a high conversion efficiency with simple meansand great reliability.

As regards a most favorable possible absorption behavior for heatradiation, it would be possible to design and construct the filament inthe form of a strip, or, quite generally, as a flat filament. As analternative thereto, one could also make the filament, quite generally,as a volume filament, i.e., a filament, which occupies a spatial volume,or comprises a volume. In particular, one could make the filament in theshape of a cup or cylinder jacket. In this connection, a configurationas a complete cylinder jacket or even as a portion thereof, inparticular a cylinder jacket half is possible. In the case of asubstantially complete cylinder jacket, such a jacket could also be madeopen on its side or axially slotted. This is favorable with respect tothe thermal expansion behavior of the filament.

To guarantee a particularly effective absorption of the heat radiationbeing reflected from the inner side of the bulb, the diameter of thecylinder jacket, or of a portion or half thereof, could be only slightlysmaller than the diameter of the bulb. In particular in this instance,it would be possible to arrange the filament in the bulb in concentricand/or coaxial relationship with a longitudinal axis of the bulb.

Depending on its configuration, the filament could divide the interiorof the bulb into one or more half spaces or subspaces.

The bulb could have such a large outer surface that it is possible todissipate the surface heat, which is generated, for example, byabsorption of heat radiation, with the use of convection cooling or anyother forced cooling. The size and form of the filament and the bulbcould adapted to each other in a corresponding manner.

Basically, the filament could contain tungsten, and/or rhenium, and/ortantalum, and/or zirconium, and/or niobium. In this connection,adjustments are to be made to the respective needs of the light sourcecharacteristics. The filament could contain the last-mentioned materialsin a sintered form.

Furthermore, the filament could be composed at least in part of anonmetal. This could improve the mechanical stability of the filament.

With respect to very high surface temperatures, and very high lightcurrents in the visible range, the filament could be composed at leastin part of tantalum carbide, and/or rhenium carbide, and/or niobiumcarbide, and/or zirconium carbide. This would allow to reach surfacetemperatures, which are higher than is normal for known tungstenfilament lamps.

Concretely, the heating element could be an incandescent element that isheated by the electric current. The filament is heated by the heatradiation of the incandescent element. The incandescent element may beadapted to the required lamp output independently of the filament. In aparticularly simple manner, the incandescent element could be a heatingcoil.

As regards a particularly favorable heating of the filament by theincandescent element, the latter could be arranged within a space orhalf space formed by the filament, preferably within a cylinder jacketor a cylinder jacket half. In this connection, quasi the largest portionof the heat radiated from the incandescent element is absorbed by thefilament. When the filament is designed as a body that is open insections—for example, as a cylinder jacket half—the incandescent elementwill be able to contribute in addition to the generation of light. Inthis instance, the incandescent element radiates in the direction, whichis predetermined by the configuration of the filament. The light sourcewould be able to emit light already before the filament is heated to thetemperature necessary for the light emission. A time delay between theactivation of the light source and light emission is thus largelyavoided.

In a particularly simple manner, the incandescent element could beformed from tungsten. In this instance, the use of conventional tungstenheating coils is possible.

In a constructionally very simple manner, the filament could be attachedto a power supply conductor for the heating element or incandescentelement, thereby avoiding additional holding means for the filament inthe bulb.

As an alternative or in addition to a heating of the filament by meansof a heated incandescent element, one could arrange magnetic inductorsin the bulb or outside thereof for an indirect heating of the filament.Likewise with that, an indirect heating of the filament is possible in asimple manner.

To optimize the reflection behavior of the inner side of the bulb, whichis transparent for the visible light, the bulb could have a mirrorcoating on its inner side. In a particularly favorable manner, samecould be a dielectric multilayer coating. With that, a spectrallyselective mirror coating is present, which largely reflects the portionof heat radiation and transmits the portion of visible radiation.

In the case of a filament, which does not fully surround an incandescentelement, heat radiation is also emitted from the incandescent elementdirectly to the inner side of the bulb. From this inner side, the heatradiation in turn is reflected on the filament.

Likewise, the heat radiation emitted from the filament is reflected fromthe inner side of the bulb, and thus contributes to the backheating ofthe filament. As a whole, the light source of the present inventioncould be described a radiation furnace lamp, wherein the bulb forms aninternally heated radiation furnace for the infrared radiation.

The large, possible surface of the filament permits constructing lightsources with high light outputs. It is likewise possible to adjust thecolor temperature of the light source independently of the surfacetemperature of the filament or incandescent element. This may occur bythe spectrally selective mirror coating, which is capable ofpredetermining the transmitted spectral distribution of the radiationoutput emitted from the bulb and thus the color temperature.

In comparison with previous light sources of the same light output, itis possible to lower in particular the surface temperature of both theincandescent element and the filament, inasmuch as, on the one hand, theentire radiation output of the incandescent element must correspond onlyto the sum of the visible radiation output and the thermal dissipationpower of the light source. However, same is smaller by the portion ofreflected and reabsorbed heat radiation or portion of the infraredradiation output than the total radiation output of comparabletemperature radiators of the art. In accordance with theStefan-Boltzmann law, the total specific heat radiation is a function ofthe temperature, so that the incandescent element of the light sourceaccording to the invention can be operated at a lower temperature incomparison with the directly heated filament of comparable thermal lightsources of the art. On the other hand, likewise for comparison, thesurface temperature of the filament may be adjusted lower, since thecomparable visible light current can be generated by a larger and coldersurface of the filament. In this connection, the filament surface formsa new, additional constructional degree of freedom.

While it is possible to operate the filament at a relatively lowtemperature, and while with that also a relatively low evaporation ofthe filament material is reached, a disturbing evaporation may occurbecause of the very large surface, which is as close as possible to theinner side of the bulb. As a result of filament material, which hasevaporated and settled on the inner side of the bulb, the reflectivityof the inner side of the bulb or the mirror coating on the inner side ofthe bulb is reduced, and the absorption of the bulb or the mirrorcoating and the thermal dissipation respectively are increased. It istherefore desirable to minimize the evaporation of the filament materialto the greatest extent.

For minimizing the evaporation of the filament material, the bulb couldcontain an inert gas and/or a halogen gas, with the halogen gascontaining bromine and/or iodine. With that, it would be possible togenerate a normal tungsten iodide circulation in the case of a tungstenfilament.

An alternative solution to the evaporation problems could occur bycoating the filament and/or the incandescent element with a coatingmaterial, which has a higher melt point than the material of thefilament and/or incandescent element. This lies in the dependency of thetemperature-dependent vapor pressure of a solid from its melt point.Furthermore, the deposit of the coating material could exhibit a lesserabsorptivity than the deposit of the standard filament material or thematerial of the incandescent element. As a coating material with a veryhigh melt point, it would be possible to use, for example, tantalumcarbide, and/or rhenium carbide, and or niobium carbide, and/orzirconium carbide.

As a result of the constructionally necessitated large filament surface,it is possible to generate very high light currents and to emit themfrom the light source, so as to enable an illumination of large buildinginteriors or outdoor areas with only one light source according to theinvention.

There exist various possibilities of improving and further developingthe teaching of the present invention in an advantageous manner. To thisend, one may refer to the following detailed description of a preferredembodiment with reference to the drawing. In conjunction with thedetailed description of the preferred embodiment of the invention withreference to the drawing, also generally preferred improvements andfurther developments of the teaching are described.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective side view of the embodiment of a light sourceaccording to the invention;

FIG. 2 is a top view of the embodiment of FIG. 1; and

FIG. 3 is a schematic circuit diagram illustrating the series connectionof the heating element and filament to the power supply.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a perspective side view of an embodiment of a light sourceaccording to the invention. The light source is designed and constructedas an incandescent lamp, which comprises a bulb 1 that accommodates afilament 2. For heating the filament 2, a heating device 3 is provided,which provides an electric current. The heated filament 2 emits bothvisible light and heat radiation. The temperature of the heated filament2 can be about 3,000 degrees Celsius.

With respect to a high conversion efficiency and a reliable operation ofthe light source, the heating device 3 includes a heating element 4 forindirectly heating the filament 2. The heating element 4 is anincandescent element in spiral form, and may consist, for example, oftungsten. The filament 2 is realized substantially in the shape of acylinder jacket, and therefor has a large absorption surface for aradiation of heat, which is reflected from the inner side of bulb 1. Asa result, the filament 2 is effectively backheated by the reflected heatradiation. This makes it possible to select a lower temperature of theheating element 4 than would be necessary in the case of a conventionallight source with the same light output. Consequently, it is possible tooperate the light source of the present invention with lesser energy andthus more economically than conventional light sources.

The filament 2, which is in the form of a cylinder jacket, is attachedin a simple manner to a power supply conductor 5 and in series with theheating element 4 as illustrated in FIG. 3. The heating element 4 orincandescent element in the form of a spiral is positioned in concentricand coaxial relationship with the filament 2. The filament 2 in turn isarranged in the bulb 1 in concentric and coaxial relationship with quasitubular bulb 1. The filament 2 having the shape of a cylinder jacket ortube is made from tungsten.

In the lower end of bulb 1, electrical contacts 6 are provided forsupplying a current. The electrical contacts 6 are fused together withthe lower end of bulb 1.

The diameter of filament 2 is only slightly smaller than the diameter ofbulb 1.

The inner side of bulb 1 is provided with a mirror coating 7. The mirrorcoating 7 is used for an effective reflection of the heat radiation thatis emitted from heating element 4 and/or filament 2.

The heating element 4 and/or the filament 2 could include a coating of amaterial with a very high melt point. This would allow to reduce anevaporation of filament material and/or heating element material.

FIG. 2 is a top view of the embodiment of FIG. 1. As best seen in thisFigure, the filament 2 is arranged in bulb 1 in substantially concentricrelationship, and the heating element 4 is positioned in filament 2substantially in the center thereof.

As regards further advantageous improvements and further developments ofthe teaching in accordance with the invention, the general part of thedescription on the one hand and the attached claims on the other areherewith incorporated by reference.

Finally it should be expressly emphasized that the foregoing, merelyarbitrarily selected embodiment is used only for explaining the teachingof the present invention, without however limiting same to thisembodiment.

What is claimed is:
 1. A light source comprising a bulb, a filamentmounted within said bulb and which has an arcuate configuration whenviewed in plan so as to define a space within the bulb which is at leastpartially enclosed by the filament, an electrical heating device forheating the filament whereby the filament can be heated to cause theemission of visible light and heat radiation, said heating deviceincluding an incandescent heating element positioned within said spacefor indirectly heating the filament, and wherein said heating devicefurther comprises an electrical circuit connecting the filament and theheating element in series.
 2. The light source of claim 1 wherein saidheating device further includes a pair of electrical contacts which areelectrically connected to said heating element.
 3. The light source ofclaim 1 wherein said filament is in the form of at least a portion of acylindrical jacket.
 4. The light source of claim 3 wherein the at leasta portion of a cylindrical jacket includes a lengthwise extendingopening.
 5. The light source of claim 3 wherein the at least a portionof a cylindrical jacket extends for at least 180° when viewed in planand defines a diameter which is only slightly smaller than a diameterdefined by the bulb.
 6. The light source of claim 1 wherein the bulbdefines a longitudinal axis, with the filament being configured sodefine a coaxial center axis.
 7. The light source of claim 1 wherein thebulb defines a longitudinal axis and wherein the heating element is inthe form of a helical coil which is disposed coaxially along thelongitudinal axis.
 8. The light source of claim 1 wherein the filamentcomprises a sintered metal selected from the group consisting oftungsten, rhenium, tantalum, zirconium, niobium, and mixtures thereof.9. The light source of claim 1 wherein the filament includes a nonmetal.10. The light source of claim 1 wherein the filament comprises a metalselected from the group consisting of tantalum carbide, rhenium carbide,niobium carbide, zirconium carbide and mixtures thereof.
 11. The lightsource of claim 1 wherein the heating element essentially comprisestungsten.
 12. The light source of claim 1 wherein the bulb includes aninner surface which includes a mirror coating.
 13. The light source ofclaim 12 wherein the mirror coating comprises a dielectric multilayercoating.
 14. The light source of claim 13 wherein the dielectricmultilayer coating is spectrally selective so as to substantiallyreflect the heat radiation emitted by the filament while substantiallytransmitting the emitted visible light.
 15. The light source of claim 1wherein the bulb is at least partially filled with an inert gas and/or ahalogen gas.
 16. The light source of claim 1 wherein the bulb is atleast partially filled with a halogen gas which contains bromine and/oriodine.
 17. The light source of claim 1 wherein the filament and/or theheating element are coated with a coating material which has a highermelt temperature than the material upon which it is coated.
 18. Thelight source of claim 17 wherein the coating material includes a carbideselected from the group consisting of tantalum carbide, rhenium carbide,niobium carbide, zirconium carbide, and mixtures thereof.